Blink detection apparatus

ABSTRACT

A blink detection apparatus includes an eyelid state index value measurement unit configured to chronologically measure an eyelid state index value, and a signal processing device. The signal processing device is configured to determine that a blink occurs when a change width of the measured eyelid state index value from a baseline that is a reference value of the eyelid state index value in a case where eyelids are in an opened state exceeds a magnitude of a threshold from the baseline, to count the number of times when the change width of the measured eyelid state index value from the baseline exceeds the magnitude of a tentative threshold from the baseline, and to set the threshold to a tentative threshold at which change in the number of times with respect to the tentative threshold is minimal.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to Japanese Patent Application No.2017-167643 filed on Aug. 31, 2017, which is incorporated herein byreference in its entirety including the specification, drawings andabstract.

BACKGROUND 1. Technical Field

The present disclosure relates to an apparatus which detects blinks(eyeblinks) of a human, and more in detail, relates to an apparatuswhich detects blinks in a waveform indicating an eyelid state in anopening-closing motion of eyelids in an electrooculogram (EOG) or thelike of a human.

2. Description of Related Art

There are proposed technologies for sensing blinks of a human for thepurpose of sensing sleepiness of the human and similar purposes since ablink occurrence frequency of a human and its change are related to thedegree of sleepiness of the human. As methods for detecting blinks of ahuman, there is known an EOG method (electrooculography method) in whichthe vicinity of an eye is equipped with electrodes and a potentialdifference (ocular potential) between the retina and the cornea inresponse to opening-closing of eyelids is detected as well as a methodof capturing an image of eyelids of the human and detectingopening-closing of the eyelids from the captured image. In the EOGmethod, briefly stated, since when an opening-closing motion of theeyelids occurs, as schematically drawn in FIG. 6A, temporary change ofthe ocular potential occurs, such ocular potential change is obtainedfrom chronologically measured ocular potential data, and thereby, ablink is sensed. Regarding this point, “Automatic Detection of Eyeblinksand Analysis of Eyeblink Waveforms (A Computerized Identification andData Analysis of Eyeblink EOG Waves)” Hiroaki YUZE; Hideoki TADA,Ergonomics (Japan Ergonomics Society), Vol. 30, No. 5, pp. 331-337proposes, as an algorithm for automatically detecting such ocularpotential change due to eyeblinks, to calculate a differential value ofchronological data of the ocular potential, and as shown in FIG. 6B, todetect, as a blink, a waveform part in which the value consecutivelyexceeds thresholds on the negative side and the positive side within apredetermined time (approximately 0.2 seconds) in the waveform of thedifferential value of the ocular potential. Japanese Patent ApplicationPublication No. 2017-42269 proposes, in an apparatus which senseseyeblinks in a waveform of chronological data of the differential valueof the ocular potential and which detects, as a eyeblink waveform,change of the differential value of the ocular potential when thedifferential value of the ocular potential changes from an upperthreshold to a lower threshold within a predetermined time after itexceeds the upper threshold or when it changes from the lower thresholdto the upper threshold within a predetermined time after it goes belowthe lower threshold, to improve detection accuracy of the eyeblinkwaveform using, as the upper threshold and the lower threshold, numbersobtained by multiplying coefficients for the thresholds whichcoefficients are set based on frequency characteristic values indicatingfrequency characteristics of the differential value of the ocularpotential by a standard deviation of the differential value of theocular potential.

SUMMARY

In the technologies of detecting blinks as above, there is generallymade a configuration to chronologically measure an index valueindicating an eyelid state or position in an opening-closing motion ofeyelids using a sensor for detecting an ocular potential, a camera forcapturing an image of the eyelids, or the like, and to determine that ablink occurs when change of the index value at the time when the eyelidsare closed relative to the index value (baseline) at the time when theeyelids are opened exceeds a “threshold” (that is, when the index valueis displaced upward beyond the “threshold” in the case where the indexvalue at the time when the eyelids are closed is displaced upward fromthe index value (baseline) at the time when the eyelids are opened, orwhen the index value is displaced downward beyond the “threshold” in thecase where the index value at the time when the eyelids are closed isdisplaced downward from the index value (baseline) at the time when theeyelids are opened). In such blink detection technologies, as to settingof the “threshold” which is a criterion for blink occurrence, typicallyin advance, visual observation of an opening-closing motion of eyelidsis collated with change of the index value, and a value beforehandspecified as the index value at the time when the eyelids are changedfrom the opened state to the closed state is set as the “threshold” tobe used for actual detection of blinks of a subject, for example, in thecase of detecting blinks of a driver who is driving a vehicle

However, in the case where the threshold preset as above is fixed to bea certain value, even if it is optimal immediately after the start ofblink detection processing, there can be a case where it becomesincapable of being used for correct detection of blink occurrence if thebaseline which is the reference of the index value varies or a changewidth of the index value in blink occurrence varies due to a factor ofchange in physical conditions of the subject and various other factorsafter actual blink detection processing for the subject is started.Accordingly, to prepare for such a case, it is useful that a device fordetecting blinks has a configuration capable of appropriately resettingthe threshold even after actual blink detection processing for a subjectis started.

Therefore, the present disclosure provides a configuration capable ofsetting an appropriate threshold even after the start of blink detectionprocessing in a blink detection apparatus which chronologically measuresan index value indicating an eyelid state in an opening-closing motionof eyelids and determines that a blink occurs when the index value isdisplaced upward or downward beyond the threshold.

Moreover, in the apparatus as above, setting of the threshold after thestart of blink detection processing is performed in appropriate timing.Therefore, the present disclosure provides an apparatus as above that,is configured to be capable of setting the threshold in proper timing.

A blink detection apparatus according to an aspect of the presentdisclosure includes an eyelid state index value measurement unitconfigured to chronologically measure an eyelid state index valueindicating a state between an opened state and a closed state of eyelidsin an eye of a subject, and a signal processing device. The signalprocessing device is configured to determine that a blink occurs when achange width of the measured eyelid state index value from a baselinethat is a reference value of the eyelid state index value in a casewhere the eyelids of the subject are in the opened state exceeds amagnitude of a threshold from the baseline, to store the chronologicallymeasured eyelid state index value, to count, with respect to each of aplurality of tentative thresholds, magnitudes of the tentativethresholds being different from one another, the number of times whenthe change width of the eyelid state index value from the baselineexceeds a magnitude of the tentative threshold from the baseline, forthe eyelid state index values that are chronologically measured andstored over a predetermined period, and to set the threshold to atentative threshold at which change in the number of times with respectto the tentative threshold is minimal.

In the aforementioned aspect, the “eyelid state index value” may betypically an ocular potential, in this case, the “eyelid state indexvalue measurement unit” is configured to be an “ocular potentialmeasurement unit” which measures a potential difference between at leastpair of electrodes with which the vicinity of an eye is equipped suchthat a potential difference between the retina and the cornea inresponse to opening-closing of the eyelids can be measured in anyaspect, and a signal of the potential difference between the electrodes,that is, the ocular potential undergoes AC-to-DC conversion and is usedfor processing after that. In this case, it is determined, based on thearrangement of the electrodes on a subject, to which of the positiveside and the negative side change of the ocular potential is orientedrelative to the value of the ocular potential in the case where theeyelids are in the opened state when a blink occurs, that is, when theeyelids become from the opened state to the closed state. It should beunderstood that this aspect includes the case where the change of theocular potential at the time when a blink occurs is on the positiveside, and also, the case where it is on the negative side. Notably, the“eyelid state index value” may be a time differential value of theocular potential. Moreover, in another aspect, the “eyelid state indexvalue” may be an index value indicating the degree of opening of eyelids(the distance between the upper eyelid and the lower eyelid or theposition of the upper eyelid relative to the lower eyelid) in an imageof an eye of a subject. In this case, the “eyelid state index valuemeasurement unit” may include a camera which captures an image of theeye of the subject, and detecting the degree of opening of the eyelidsby detecting the positions of the upper eyelid and the lower eyelid inthe image of the eye by an arbitrary technique for image processing. Itshould be noted that in this case, to which of the positive side and thenegative side change of the degree of opening of the eyelids in the casewhere the eyelids become from the opened state to the closed state isoriented depends on the definition of the degree of opening of theeyelids, and in this aspect, includes the case where the change of thedegree of opening of the eyelids at the time when a blink occurs is onthe positive side, and also, the case where it is on the negative side.Notably, the “eyelid state index value” may be a time differential valueof the degree of opening of eyelids. The important thing is that the“eyelid state index value” is displaced when the eyelids become from theopened state to the closed state.

Furthermore, in the aforementioned configuration, the “baseline” may bea reference value of the eyelid state index value in the case where theeyelids of a subject are in the opened state as above, and typically,may be a time average value or the like of the eyelid state index valuesmeasured in the case where the eyelids are in the opened state. Here,the eyelid state index values measured in the case where the eyelids arein the opened state may be normally eyelid state index values beforehandmeasured during a period when the eyelids are visually confirmed to bein the opened state. Notably, when the eyelid state index value is anocular potential or its time differential value, the baseline may besimply set to be 0 V as the reference value of the eyelid state indexvalue in the case where the eyelids are in the opened state, and itshould be understood that this case also belongs to the scope of thisaspect. As above, the aforementioned eyelid state index value istypically maintained to be close to the baseline when the eyelids are inthe opened state, and when the eyelids become in the closed state, thatis, when a blink occurs, changes upward (to the positive side) ordownward (to the negative side) beyond the baseline by a certain width.Moreover, the “magnitude of the threshold from the baseline” correspondsto a magnitude of the difference between the baseline and the threshold.Accordingly, in the apparatus of this aspect, it can be determined thata blink occurs when the change width of the eyelid state index valuefrom the baseline exceeds the magnitude of the threshold from thebaseline as above. Notably, in an embodiment, since when the eyelidstate index value is displaced from the baseline to the positive side inblink occurrence, the change width of the eyelid state index value fromthe baseline exceeds the magnitude of the threshold from the baselinewhen the eyelid state index value exceeds the threshold, it may bedetermined that a blink occurs when the eyelid state index value exceedsthe threshold. Moreover, since when the eyelid state index value isdisplaced from the baseline to the negative side in blink occurrence,the change width of the eyelid state index value from the baselineexceeds the magnitude of the threshold from the baseline when the eyelidstate index value goes below the threshold, it may be determined that ablink occurs when the eyelid state index value goes below the threshold.

However, as having already been stated, as to the “threshold” which isthe criterion for blink occurrence in the configuration of the blinkdetection apparatus as in this aspect above, there can be a case wherein the state where it remains to be fixed to be a certain value, blinkoccurrence cannot be detected with excellent accuracy when the baselinevaries or the change width of the index value in blink occurrencevaries. More in detail, while as to the eyelid state index value, aslong as an abnormal event does not occur, when a blink occurs, theeyelid state index value is displaced from the baseline at its maximumby an approximately fixed width (normally, the eyelid state index valuedocs not vary largely away from the value in blink occurrence), as tosuch an eyelid state index value which varies as above, when themagnitude of the threshold from the baseline is too small relative tothe change width of the eyelid state index value from the baseline whichchange width corresponds to blink occurrence, this results in a falsedetection of regarding the change of the eyelid state index value of noblink occurrence as being a blink, and when the magnitude of thethreshold from the baseline is too large, this results in a misdetectionof blink occurrence.

Therefore, the aforementioned apparatus of this aspect enables update ofthe threshold to be a proper value in proper timing as above.Specifically, as above, the “signal processing device” is configured tostore the chronologically measured eyelid state index value, to count,with respect to each of the plurality of tentative thresholds,magnitudes of the tentative thresholds being different from one another,the number of times when the change width of the eyelid state indexvalue from the baseline exceeds the magnitude of the tentative thresholdfrom the baseline (magnitude of the difference between the baseline andthe tentative threshold) over a predetermined period, and to set thethreshold to the tentative threshold at which the change in the numberof times with respect to the tentative thresholds is minimal. That is,in the “signal processing device”, first, the number of times when thechange width of the eyelid state index value from the baseline exceedsthe magnitude of the tentative threshold from the baseline, so to speak,“the number of times of tentative blink occurrence” is counted for eachtentative threshold while the tentative threshold being changed in theeyelid state index values recorded over the predetermined period, next,the tentative threshold at which the change in the number of times oftentative blink occurrence is minimal is detected, this tentativethreshold is updated for the “threshold” which is the criterion forblink occurrence.

According to the configuration of the signal processing device above,even after the start of blink detection processing, the “threshold” canbe updated to be a more appropriate value using the eyelid state indexvalues having been measured so far. That is, in the configuration ofdetecting blink occurrence by the change width of the eyelid state indexvalue from the baseline exceeding the magnitude of the threshold fromthe baseline as in this aspect, as previously touched on, when themagnitude of the threshold from the baseline is too small, this causesmany false detections of blink occurrence, and when the magnitude of thethreshold from the baseline is too large, this causes many misdetectionsof blink occurrence, but when the “threshold” is the proper value, ifcan be considered that both false detections and misdetections of blinkoccurrence are reduced and the number of times detected is stabilized.Accordingly, the “threshold” that the number of times detected isstabilized at, that is, the tentative threshold at which the change inthe number of times of tentative blink occurrence is minimal can beemployed as the proper “threshold”. Actually, as described in thesection of embodiments later, it is indicated that blinks can bedetected with excellent, accuracy when the “threshold” selected as aboveis used.

In the aforementioned aspect, the signal processing device may beconfigured to count, with respect to each tentative threshold, thenumber of times when the change width of the eyelid state index valuefrom the baseline exceeds the magnitude of the tentative threshold fromthe baseline while the tentative threshold is changed or the tentativethreshold is scanned at every predetermined width. In this case, in thesignal processing device, in an aspect, a tentative threshold at which adifference in the number of times of change of the tentative thresholdby the predetermined width is minimal may be selected as the thresholdamong the tentative thresholds. Moreover, in the signal processingdevice, in another aspect, a tentative threshold at which a standarddeviation in the number of times with respect to a predetermined number(for example, five) of the adjacent tentative thresholds, is minimal maybe selected as the threshold among the tentative thresholds. The pointis that in the case of generating a histogram of the number of timeswhen the change width of the eyelid state index value from the baselineexceeds the magnitude of the tentative threshold from the baseline withrespect to the tentative threshold, the tentative threshold that thehistogram is flattest at is sufficient to be selected as the threshold.

Moreover, in the aforementioned aspect, the “predetermined period” forthe eyelid stale index value used for setting the threshold may bearbitrarily set by a designer or a user of the apparatus. For example,the “predetermined period” may be the length of a predetermined time. Inthis case, the “length of the predetermined time” may be act to be thelength of time during which a quantity of eyelid state index values withwhich setting of the threshold can be achieved with excellent accuracyare included. Otherwise, it may be a period until the number of times ofblink occurrence that is determined by the signal processing devicereaches a predetermined number of times. Here, the “predetermined numberof times” may be the number of times of blinks with which setting of thethreshold can be achieved with sufficient accuracy. Furthermore, in theaforementioned aspect, the signal processing device may be configured toreset the threshold for each elapse of the predetermined period, and maybe configured to be able to update the threshold in proper timing.

As above, in the aforementioned aspect, in the blink detection apparatuswhich chronologically measures the eyelid state index value anddetermines that a blink occurs when the change width of the eyelid stateindex value from the baseline exceeds the magnitude of the thresholdfrom the baseline, when a proper threshold is set, false detections andmisdetections of blink occurrence are reduced, and based on thestatistical knowledge that in the vicinity of the proper threshold, thecounted number of times of blink occurrence is stabilized, the thresholdcan be updated to be a proper value. According to such an aspect, sincewhen detection of blinks of a driver who is driving a vehicle, detectionof blinks of a subject who is working at a desk, or the like isperformed, the threshold can be updated to be a proper value in propertiming even when a baseline varies or a change width of an index valuein blink occurrence varies after the start of blink detectionprocessing, more excellently accurate detection of blinks can beachieved. Moreover, according to this aspect, since the thresholdupdating processing can be performed in parallel during performance ofblink detection processing, the blink detection processing is not neededto be suspended due to the update of the threshold, and detection ofblinks can be performed consecutively for a long time.

Other objects and advantages of this aspect will be apparent from thefollowing description of embodiments of the present disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

Features, advantages, and technical and industrial significance ofexemplary embodiments will be described below with reference to theaccompanying drawings, in which like numerals denote like elements, andwherein:

FIG. 1A is a diagram schematically showing change of an ocular potentialduring blink occurrence obtained by an EOG method;

FIG. 1B is a diagram schematically showing a configuration of a blinkdetection apparatus of the present disclosure;

FIG. 2A is a diagram schematically showing a situation of counting thenumber of times of tentative blinks (the number of times when an eyelidstate index value exceeds a tentative threshold) for each magnitude ofthe tentative threshold with respect to the eyelid state index value(ocular potential) recorded over a predetermined period in accordancewith teachings of the present disclosure;

FIG. 2B shows a histogram (bar graph) of the number of times oftentative blinks which histogram is obtained by plotting the numbers oftimes of tentative blinks with respect to the tentative thresholds, acurve ft presenting the number of times of tentative blinks, and a curveΔft presenting change in the number of times of tentative blinks withrespect to the tentative threshold;

FIG. 2C shows a typical example of time change of the ocular potential,and exemplarily showing that blinks can be detected with excellentaccuracy when a tentative threshold at the minimum value in the changein the number of times of tentative blinks with respect to the tentativethreshold in FIG. 2B is set to be a threshold;

FIG. 3A shows a situation of collating blink occurrence visuallyconfirmed with blink occurrence determined from the ocular potentialmeasured by a sensor. In the figure, “OK” denotes that the blinkoccurrence visually confirmed can also be detected from the ocularpotential (correct answer), and “NG” denotes that the blink occurrencevisually confirmed cannot be detected from the ocular potential of thesensor (misdetection) and that a blink is erroneously detected from theocular potential of the sensor (false detection);

FIG. 3B is a diagram obtained by plotting detection rates with respectto false detection rates, the detection rate (ratio of the number oftimes of blinks detected from the ocular potential relative to thenumber of times of blink occurrence visually confirmed) and the falsedetection rate (ratio of the number of times of false detectionsrelative to the number of times of occasions of being detected as blinksfrom the ocular potential) being calculated, while the tentativethreshold for the ocular potential measured by the sensor being changed.The tentative threshold of the plot nearest from the point with 1.0 ofdetection rate and 0 of false detection rate is an optimal threshold;

FIG. 4 is a diagram showing, in the form of a flowchart, an example ofblink detection processing in the present embodiment;

FIG. 5A is a diagram showing, in the form of a flowchart, an example ofthreshold setting monitoring processing in the present embodiment;

FIG. 5B is a diagram showing, in the form of a flowchart, an example ofthreshold setting processing in the processing of FIG. 5A;

FIG. 6A is a diagram schematically showing change of the ocularpotential, in blink occurrence, obtained by the EOG method in therelated art. The reason why a change direction of the ocular potentialis reverse to that in FIG. 1A is that an arrangement of a positiveelectrode and a negative electrode is reverse to that in the case ofFIG. 1A;

FIG. 6B is a diagram schematically showing a differential value of thechange of the ocular potential, in blink occurrence, obtained by the EOGmethod in the related art. The reason why a change direction of theocular potential is reverse to that in FIG. 1A is that the arrangementof the positive electrode and the negative electrode is reverse to thatin the case of FIG. 1A;

FIG. 7A is a diagram showing a typical time change of the ocularpotential along with a threshold which is a criterion of blinkdetection. There is shown an example in which although in the leftstage, ocular potential changes B in which blinks occur can be detectedwith the set threshold, in the right stage, baseline rises, and falsedetections X becomes more with the set threshold; and

FIG. 7B is a diagram showing a typical time change of the ocularpotential along with a threshold which is a criterion of blinkdetection. There is shown an example in which although in the leftstage, ocular potential changes B in which blinks occur can be detectedwith the set threshold, in the right stage, the baseline falls, andmisdetections Y becomes more with the set threshold.

DETAILED DESCRIPTION OF EMBODIMENTS

Hereafter, several embodiments will be described in detail withreference to the appended drawings. In the figures, the same signsdenote the same parts.

Configuration of Apparatus

In some embodiments of a blink detection apparatus of the presentdisclosure, similarly to the case of JP 2017-42269 A, as a basicconfiguration, an ocular potential of a subject is measured fry the EOGmethod. As schematically shown in FIG. 1A, it is known that the ocularpotential changes in response to an eyelid state in an opening-closingmotion of eyelids and that a potential difference occurs to be asubstantially fixed width between the case where the eyelids are in afully opened state and the case where they are in a fully closed state(hereafter, the case where the eyelids are in the fully opened state issupposed to be stated as “opened state” and the case where the eyelidsare in the fully closed state is supposed to be stated as “closedstate”), since it can be referred to as an index value indicating theeyelid state, that is, an eyelid state index value, when a potential inthe case where the eyelids of the subject are in the opened state forchronological data of the ocular potential is set as a reference value,that is, a baseline and when a displacement width of the ocularpotential from the same exceeds a magnitude of a threshold which is setto approximately correspond to the potential difference between theopened state and the closed state or to be slightly smaller than thesame, it is determined that the eyelids of the subject become in theclosed state and that a blink occurs, and thereby, the blink can bedetected.

With reference to FIG. 1B, in a configuration of the blink detectionapparatus of the present embodiment, first, at least pair of electrodesEL1, EL2 are pasted around an eye and eyelids 2 of a face 1 of asubject, and a potential difference between the electrodes issequentially sent as an ocular potential signal to a signal processingdevice 3. In the signal processing device 3, at a “preprocessing unit”,the ocular potential signal from the electrodes sequentially undergoesprocessing such as digitalizing to be converted into a form adaptable todownstream processing, and the ocular potential signal that hasundergone the processing is sent to a “blink detection unit” and a “datastorage unit”. In the blink detection unit, the ocular potential signaland a threshold are compared with each other in an aspect describedlater, and blink occurrence is detected. Meanwhile, the ocular potentialsignal sent to the data storage unit is temporarily stored andaccumulated therein, and after that, is used by a “threshold settingunit” for setting the threshold used by the blink detection unit. Asdescribed later in detail, the threshold setting unit determines andsets the optimal threshold for detecting blink occurrence using theocular potential signal obtained over a predetermined period, and theset threshold is referred to by the blink detection unit. Moreover, inthe blink detection apparatus of the present embodiment, even duringperformance of blink detection processing, a configuration to update thethreshold at predetermined periods by the threshold setting unit may beadopted, and a “monitoring unit” for monitoring the elapse of thepredetermined period may be provided. The monitoring unit may beconfigured to refer to time from a timer and/or the number of times ofblinks detected in the blink detection unit and to give an instructionof performance of threshold setting processing (threshold setting flag)to the threshold setting unit in an aspect described later. Furthermore,the result of blink detection may be sent, for example, to an arbitrarydevice (not shown) for sleepiness determination, or may be sent, forexample, to a display (not shown) to be displayed thereon. The signalprocessing device 3 may be typically configured to be a computer device,and in the same, a CPU, a storage device, and an input-output device(I/O) which are coupled to one another via a not-shown bidirectionalcommon bus are included in a normal aspect, operation of the individualpans of the blink detection apparatus being achieved by the CPUexecuting a program.

Notably, while in FIG. 1A, as to the ocular potential used as the eyelidstate index value, a direction in which the ocular potential in theclosed state of the eyelids changes relative to the opened state of theeyelids is on a positive side, since the direction of the change isdetermined depending on an arrangement of the electrodes EL1, EL2 pastedon the face 1 of the subject, the direction in which the ocularpotential in the closed state of the eyelids changes may be on anegative side depending on the arrangement (see FIG. 6A; in such a case,the threshold is set on the lower side of the baseline). The eyelidstate index value used for blink detection may be chronological data ofocular potentials themselves as shown in the figure, or may bechronological data of time differential values of them. In this case,since change of the eyelid state index value in blink occurrence occurson either the positive side or the negative side (see FIG. 6B), thebaseline may be set to be substantially zero, and the threshold(s) maybe set on any one or both of the positive side and the negative side.Furthermore, the eyelid state index value may be a distance between anupper eyelid and a lower eyelid obtained by detecting positions of theupper eyelid and the lower eyelid by an arbitrary image processingtechnique from an image of an eye of a subject which image is capturedby a camera (not shown). In this case, in the post processing unit,processing until measurement of the distance between the upper eyelidand the lower eyelid from the camera image is performed, chronologicaldata of the distances between the upper eyelid and lower eyelid is sentto the “blink detection unit” and the “data storage unit”. In the casewhere the distance between the upper eyelid and the lower eyelid is usedas the eyelid state index value, the distance in the opened state of theeyelids is set as the baseline, and the threshold is set to be thedistance (=0) corresponding to the closed state of the eyelids or avalue slightly larger than the same. It should be understood that any ofthe cases belongs to the scope of the present disclosure.

Principles of Detection of Blink and Setting of Threshold

In blink detection by the apparatus of the present embodiment, as above,when the change width of the sequentially measured eyelid state indexvalue from the baseline exceeds the magnitude of the threshold from thebaseline, blink occurrence is determined. Regarding this point, since inthe ocular potential or the like of a subject which ocular potential isactually used as the eyelid state index value, the change width of theeyelid state index value in blink occurrence has a variation to someextent, and other than the change of the value due to a blink, changedue to another factor such as an eyeball motion also occurs, thethreshold is adjusted such that a chance that change other than a blinkis erroneously detected as a blink is as less as possible, that is, suchthat false detections are as less as possible, and such that actuallyoccurring blinks can be detected without missing them as less aspossible, that is, such that all the blinks can be detected as much aspossible with less misdetections.

Moreover, as mentioned in “SUMMARY”, even when a certain threshold canbe appropriately used for blink occurrence detection immediately afterthe start, of blink detection processing, there can be a case where itbecomes incapable of being used for blink occurrence detection withexcellent accuracy due to change in subject's physical conditionsafterward and other change in various circumstances. For example, as inwaveforms of the ocular potential exemplarily shown in FIG. 7A(waveforms in which the value in the closed state of the eyelids isdisplaced to the positive side of the value in the opened state of theeyelids), while in the left of the figure, only the waveform parts B inwhich blinks occur can be selectively detected since all the waveformparts B in which blinks occur exceed the set threshold and variations ofthe ocular potential other than these go below the threshold, when as inthe right of the figure, the baseline rises during performance of blinkdetection processing due to some factor, variations X of the ocularpotential other than blinks also exceed the threshold, end falsedetections of blinks occur. Moreover, as in waveforms of the ocularpotential exemplarily shown in FIG. 7B, while in the left of the figure,only the waveform parts B in which blinks occur can be selectivelydetected, when in the right of the figure, the baseline falls duringperformance of blink detection processing due to some factor, waveformsY which have slightly small amplitudes among the waveform parts B inwhich blinks occur go below the threshold, which causes misdetections.In order to select only the waveform parts B in which blinks occur aswith excellent accuracy as possible even in the case of suchcircumstance change during performance of blink detection processing, itcan be considered that the threshold should be updated to be a proximalvalue in proper timing without being fixed to be a certain value.

Now, it is assumed that in the waveforms of the ocular potential used asthe eyelid state index value exemplarily shown in FIG. 7A and FIG. 7B,scanning is performed from the vicinity of the baseline in a directionin which the magnitude of the threshold becomes large, during themagnitude of the threshold from the baseline being small, falsedetections are many since variations of the ocular potential other thanblinks exceed the magnitude of the threshold as well as the waveformparts B in which blinks occur. Nevertheless, afterward when themagnitude of the threshold from the baseline becomes large to someextent, variations that exceed the magnitude of the threshold from thebaseline are only the waveform parts B in which blinks occur, and evenwhen in the vicinity, the threshold somewhat increases or decreases,change in the number of times of variations exceeding the magnitude ofthe threshold from the baseline becomes little. Then, when the magnitudeof the threshold from the baseline further becomes large, even thewaveform parts B in which blinks occur do not exceed the magnitude ofthe threshold from the baseline, and misdetections become many. Thereason is that features of waveforms of the ocular potential and thelike used as the eyelid state index value are that the amplitudes of thewaveform parts B in which blinks occur are substantially uniform (evenwith some variations) even when the baseline varies, and are larger thanvariation amplitudes of the ocular potential other than blinks whichvariation amplitudes overlap with the baseline by a certain width ormore. Accordingly, focusing on the features of the waveforms of theeyelid state index value, in a waveform of the eyelid state index valueover a certain period, while the magnitude of a temporary threshold(tentative threshold) from the baseline being gradually increased, thenumber of times of exceeding the magnitude of the tentative thresholdfrom the baseline is counted, a tentative threshold at which change inthe number of times of exceeding the magnitude of the tentativethreshold from the baseline is most stable is detected, and thereby, itcan be considered that this tentative threshold can be used as a properthreshold.

Therefore, setting of the threshold in the present embodiment isperformed based on the aforementioned knowledge. Specifically, that is,as schematically shown in FIG. 2A, first, in a waveform of thechronological data of the eyelid state index value such as the ocularpotential (in the shown example, a waveform in which the value in theclosed state of the eyelids is displaced to the positive side of thevalue in the opened state of the eyelids), the tentative threshold isbeing increased from 0 mV at predetermined distances (the distances maybe set to be wide when the tentative threshold is small and large, andthe distances may be set to be fine within a range in which thetentative thresholds are supposed to be close to the proper threshold),tentative blink occurrence is determined when the magnitude of theeyelid state index value from the baseline exceeds the magnitude of thetentative threshold from the baseline for each tentative threshold, andthe number of times thereof is counted. Then, as shown in FIG. 2B, forthe number of times of tentative blink occurrence obtained for eachtentative threshold, a histogram of the numbers of times of tentativeblink occurrence with respect to the tentative thresholds is generated,a tentative threshold that gives the flattest place in a curve ftpresenting the number of times of tentative blink occurrence, that is,the minimum minΔft in a change curve Δft of the number of times oftentative blink occurrence may be selected as the proper threshold.

The flattest place in the curve ft presenting the number of times oftentative blink occurrence, or the minimum minΔft in the change curveΔft of the number of times of tentative blink occurrence may be detectedby an arbitrary technique. In one aspect, a difference Δft of the numberof times ft(a_(t)) of tentative blink occurrence, where the tentativethresholds are a₀, a₁, . . . , a_(t), . . . , a_(n), is defined asΔft=ft(a _(t))−ft(a _(t−1))the tentative threshold at that gives the minimum value minΔft of Δftmay be selected as the appropriate threshold. In another aspect, severalstandard deviations whose center is ft(a_(t)), for example, standarddeviations of ft(a_(t−2)), ft(a_(t−1)), ft(a_(t+1)), ft(a_(t+2)) may becalculated for each of all the tentative thresholds, and the tentativethreshold a₁ at which the standard deviation is smallest may be selectedas the proper threshold (since the standard deviation is smaller as thechange is smaller).

Notably, the histogram in FIG. 2B is a histogram obtained using anactual example of an ocular potential waveform in which the value in theclosed state of the eyelids as the eyelid state index value is displacedto the positive side of the value in the opened state thereof, and thethreshold giving minΔft was 76 mV. FIG. 2C is a part of the ocularpotential waveform used for generating the histogram in FIG. 2B, and asunderstood from the figure, it can be confirmed that only the waveformof blink occurrence selectively exceeds the threshold when the tentativethreshold determined by the aforementioned setting technique of thethreshold is set to be the threshold.

Furthermore, in order to investigate validity of the thresholddetermined by the aforementioned setting technique of the thresholdaccording to the present embodiment, as mentioned below, the blinkoccurrence detected from the chronological data of the ocular potentialof a subject (data in which the value was displaced from the baseline tothe positive side in blink occurrence) was collated with the blinkoccurrence visually confirmed from a video image around an eye of thesubject which video image was captured by a video camera to investigatethat a threshold properly used for detecting blink occurrence fromchronological data of an ocular potential substantially coincided withthe threshold determined by the aforementioned setting technique of thethreshold.

Specifically, the procedure of the investigation was as follows: (i) animage of surroundings of an eye of a subject was captured by a videocamera to record this video image simultaneously to measurement ofchronological data of an ocular potential of the subject; (ii) periodswhen blinks occurred (eye closed periods) were visually investigated anddetected from the recorded video image of the video camera (upper partof FIG. 3A); (iii) while the threshold being changed from 0 mV to 300 mVin the chronological data of the ocular potential of the subject,periods of exceeding the threshold were detected as periods when blinksoccurred (eye closed periods) for each threshold (lower pan of FIG. 3A);(iv) the eye closed periods detected from the chronological data of theocular potential were collated with the eye closed periods detectedthrough visual investigation of the video image of the video camera foreach threshold, when both eye closed periods were at least partiallyoverlapped with each other, a correct answer (OK) was recognized, whenthe eye closed period detected through visual investigation of the videoimage of the video camera was not detected from the chronological dataof the ocular potential, a misdetection (NG) was recognized, when theeye closed period was detected from the chronological data of the ocularpotential in no detection of the eye closed period through visualinvestigation of the video image of the video camera, a false detection(NG) was recognized, and they were individually counted (upper and lowerparts of FIG. 3A); (v) a ratio of the number of correct answers (OK)relative to the number of eye closed periods detected through visualinvestigation of the video image of the video camera was calculated as adetection rate, and a ratio of the number of false detections relativeto the number of eye closed periods detected from the chronological dataof the ocular potential was calculated as a false detection rate, foreach threshold, which means that a higher detection rate is useful and alower false detection rate is useful e; and (vi) as shown in FIG. 3B,points for the individual thresholds were plotted, where the horizontalaxis denoted the false detection rate and the vertical axis denoted thedetection rate, and furthermore, the plot point at the shortest distancefrom the point with 1.0 of detection rate and 0.0 of false detectionrate was specified. It can be understood that the threshold at the plotpoint at the shortest distance from the point with 1.0 of detection rateand 0.0 of false detection rate is a proper value at which the falsedetection rate can be suppressed as low as possible and the detectionrate can be enhanced as high as possible.

As to the chronological data of the ocular potential from which theaforementioned histogram in FIG. 2B was obtained, the threshold at theplot point at the shortest distance from the point with 1.0 of detectionrate and 0.0 of false detection rate obtained by the aforementionedinvestigation method was 75 mV. This value is substantially equal to thethreshold obtained by the threshold setting technique of the presentembodiment, 76 mV, and accordingly, the threshold obtained by thethreshold setting technique of the present embodiment could be shown tobe the proper threshold at which the false detection rate can besuppressed as low as possible, and the detection rate can be enhanced ashigh as possible.

To be understood is that the threshold setting technique of the presentembodiment is a technique obtained by focusing on the feature of thewaveform of the eyelid state index value as mentioned above, that is,the feature that the amplitudes of the waveform parts B in which blinksoccur are substantially uniform even when the baseline varies, and arelarger than variation amplitudes of the ocular potential other thanblinks which variation amplitudes overlap with the baseline by a certainwidth or more, and moreover, that processing of setting the threshold tothe tentative threshold at which change in the number of times oftentative blink occurrence is minimal can be performed even when blinkdetection processing is not suspended. Accordingly, in the apparatus ofthe present embodiment, the threshold can be updated in proper timingafter the start of blink detection processing and even duringperformance thereof.

Operation of Apparatus

In the apparatus of the present embodiment, as above, blink detectionprocessing (FIG. 4), in the “blink detection unit”, of comparing theeyelid state index value with the threshold to detect blink occurrenceand threshold setting monitoring processing (FIG. 5A), in the“monitoring unit”, for counting the timing when the threshold is updatedin the “monitoring unit” may be performed in parallel, and thresholdsetting processing (FIG. 5B) of setting the threshold in the “thresholdsetting unit” in accordance with the principle of threshold settingdescribed above may be performed in proper timing.

The blink detection processing which is shown in the form of a flowchartin FIG. 4 and performed in the “blink detection unit” may be performedevery time when the eyelid state index value is sequentially read in,after an instruction of the start of the processing by a user of theapparatus. Notably, in the example shown in the figure, the case wherethe eyelid state index value in the closed state of the eyelids risesabove the baseline (average of the eyelid state index values in theopened state of the eyelids) as in FIG. 1A and the baseline issubstantially 0 mV is described. In this case, since the eyelid stateindex value is displaced from the baseline to the positive side in blinkoccurrence, when an eyelid state index value V_(i) exceeds a thresholdV_(th), a change width of the eyelid state index value V_(i) from thebaseline exceeds the magnitude of the threshold V_(th) from the baseline(it should be understood that the skilled in the art can achievelikewise the case where the eyelid state index value in the closed stateof the eyelids falls below the baseline; notably, in this case, when theeyelid state index value V_(i) goes below the threshold V_(th), thechange width of the eyelid state index value V_(i) from the baselineexceeds the magnitude of the threshold V_(th) from the baseline).

In the process of FIG. 4, specifically, first, the eyelid state indexvalue V_(i) is read in from the preprocessing unit (step 1), and it isdetermined whether or not the threshold V_(th) has already been set(step 2). When the threshold V_(th) has not been set, the read eyelidstate index value V_(i) is recorded in the data storage unit as it is(step 7). On the other hand, when the threshold V_(th) has been set asdescribed later in detail, as to the eyelid state index value V_(i−1)read in the previous cycle and the eyelid state index value V_(i) readin the current cycle, it is determined whether or notthe eyelid state index value V_(i−1)<the threshold V_(th) andthe eyelid state index value V_(i)>the threshold V_(th)  (1)are completed (step 3). When the condition (1) is not completed, theeyelid state is regarded as no change, and the process is put forwardsoon. When the condition (1) is completed, since this indicates that theeyelid state index value exceeds the threshold and the eyelids arechanged from the opened state to the closed state (that the change widthof the eyelid state index value from the baseline exceeds the magnitudeof the threshold from the baseline), it is determined that a blinkstarts (step 4). Next, as to the eyelid state index value V_(i−1) readin the previous cycle and the eyelid state index value V_(i) read in thecurrent cycle, it is determined whether or notthe eyelid state index value V_(i−1)>the threshold V_(th) andthe eyelid state index value V_(i)<the threshold V_(th)  (2)are completed (step 5). When the condition (2) is not completed, theeyelid state is regarded as no change, and the process is put forwardsoon. When the condition (2) is completed, since this indicates that theeyelid state index value changes from the state of exceeding thethreshold to the state of going below the threshold and the eyelids arechanged from the closed state to the opened state, it is determined thatthe blink ends (step 6). Notably, in this stage, the number of times ofblinks C_(t) is incremented by one. Then, the eyelid state index valueV_(i) is recorded in the data storage unit (step 7).

According to the aforementioned series of processes, until the thresholdis set, the eyelid state index value is only recorded. Meanwhile, afterthe threshold is set, when the eyelids are still in the opened state,step 3 and step 5 are passed through with NOs. Then, when the eyelidsare changed from the opened state to the closed state and a blinkstarts, step 3 and step 5 are passed through with YES and NO,respectively, during the closed state of the eyelids continuing, step 3and step 5 are passed through with NOs, when the eyelids are changedfrom the closed state again to the opened state and the blink ends, step3 and step 5 are passed through with NO and YES, respectively, and thus,one time of blink occurrence is detected. After that, the processing isrepeatedly performed, thereby, the eyelid state index values aresequentially recorded, in addition, every time when a blink occurs, thisis detected, and the results of the number of times and the frequency ofblinks, variations of intervals of occurrence thereof, and the like maybe used for sleepiness determination and the like.

The threshold setting monitoring processing shown in the form of aflowchart in FIG. 5A may be started to be performed simultaneously tothe start of the blink detection processing in FIG. 4. In the thresholdsetting monitoring processing above, briefly stated, processing ofinstructing performance of processing of setting the threshold used fordetecting a blink mentioned above in an aspect mentioned later isperformed for each elapse of a predetermined period. In an aspect, ofthe threshold setting monitoring processing, specifically, first, it isdetermined whether or not the threshold V_(th) has already been set(step 11), in the stage when the threshold V_(th) has not been set, athreshold setting time T_(CHECK) corresponding to a recording time ofthe eyelid state index value used for setting the threshold is set (step12), and a measurement start time is recorded in T_(set) (step 13).After that, time T_(timer) of the timer is referred to, the process isstanding by untilT _(timer) −T _(set) >T _(CHECK)  (3)is completed (step 14), and during this, the eyelid state index valueV_(i) is recorded in the blink detection processing of FIG. 4. Notably,when the eyelid state index value is the ocular potential as above, thethreshold setting time T_(CHECK) may be, for example, 2200 seconds orthe like. Then, when the condition (3) is completed, as described later,performance of the setting processing of the threshold V_(th) isinstructed (step 15), when the threshold V_(th) has been set, thethreshold setting time is recorded in T_(set) (step 16), and the setthreshold V_(th) is used in the blink detection processing in FIG. 4.Moreover, when the threshold V_(th) has been set, after that, in thethreshold setting monitoring processing, the time T_(timer) of the timermay be referred to, performance of the setting processing of thethreshold V_(th) may be instructed every time when the condition (3) iscompleted, thereby, a new threshold V_(th) may be sequentiallydetermined, and the threshold used for the blink detection processing inFIG. 4 may be sequentially updated.

The threshold setting processing shown in the form of a flowchart inFIG. 5B is performed in response to the instruction of performance (step15) of the setting processing of the threshold V_(th) by the thresholdsetting monitoring processing. In the threshold setting processing,first, an initial value V_(so) is set to a tentative threshold V_(s)(step 21), and for the eyelid state index values V_(i) sequentiallyaccumulated in the data storage unit during the period when thethreshold setting time T_(CHECK) elapses in FIG. 5A, as the number oftimes of tentative blinks, the number of times when the eyelid stateindex value V_(i) exceeds the tentative threshold V_(s), that is, thenumber of times N_(s) whenV_(i−1)<V_(s) and V_(i)>V_(s)  (4)is completed is counted and recorded (step 22). Notably, in the exampleshown in the figure, since the eyelid state index value is displacedfrom the baseline to the positive side in blink occurrence, when theeyelid state index value V_(i) exceeds the tentative threshold V_(s),the change width of the eyelid state index value V_(i) from the baselineexceeds the magnitude of the tentative threshold V_(s) from thebaseline. After that, until the tentative threshold V_(s) exceeds an endvalue V_(se) (>V_(so)) (step 23), while the tentative threshold V_(s) isincreased at every predetermined value ΔV_(s) (step 24), the number oftimes N_(s) when the condition (4) is completed is counted and recordedfor each tentative threshold V_(s) (step 22). Notably, when the eyelidstate index value V_(i) is the ocular potential, for example, settingsmay be V_(se)=0 mV; ΔV_(s)=1 mV to 10 mV; V_(se)=300 mV, and the like.Then, when V_(s)≥V_(se) is completed, as stated in the descriptionregarding FIG. 2B, a histogram of the number of times N_(s) of tentativeblink occurrence for the tentative threshold V_(s) may be generated, thetentative threshold V_(ss) at which the change in the number of timesN_(s) of tentative blink occurrence is minimal may be selected by any ofthe aforementioned techniques (step 25), and the detected tentativethreshold V_(ss) may be set as the new threshold V_(th) (step 26) andmay be used for the blink detection processing in FIG. 4.

Now, regarding the performance of the setting processing of thethreshold V_(th) for every elapse of the threshold setting timeT_(CHECK) in the aforementioned threshold setting monitoring processingof FIG. 5A, the frequencies of blinks depend on individual differencesor differences in a person, and the numbers of times of blink occurrencefor the threshold setting time T_(CHECK) also depend on individualdifferences or differences in a person. Meanwhile, in the process ofgenerating the histogram of the number of times N_(s) of tentative blinkoccurrence for the tentative threshold V_(s) in the chronological dataof the eyelid state index value, and selecting the tentative thresholdV_(ss) at which the change in the number of times N_(s) of tentativeblink occurrence is minimal, accuracies of the histogram and theselected tentative threshold V_(ss) are better as the number of times ofblink occurrence in the chronological data of the eyelid state indexvalue which is referred to is more, and in order to attain thesufficient accuracies, it is useful that blinks more than apredetermined number of times occur in the chronological data of theeyelid state index value which is referred to. Accordingly, in thethreshold setting monitoring processing of FIG. 5A, in place of updatingthe threshold V_(th) for each elapse of the threshold setting timeT_(CHECK), the threshold V_(th) may be updated every time when thenumber of times of blink occurrence exceeds a predetermined number. Itshould be noted that in the case of this aspect, it is needed that thenumber of times of blink occurrence can be detected with good accuracyto some extent before update of the threshold V_(th) (for example, thereis a possibility that it takes a very long time that the number of timesof blink occurrence, exceeds the predetermined number when the thresholdis too large and blinks can hardly be detected). Therefore, when thethreshold is set at first, the threshold setting processing may beperformed in the stage after the threshold setting time T_(CHECK)elapses, so that the threshold is once set, and after that, thethreshold V_(th) may be updated every time when the number of times ofblink occurrence exceeds the predetermined number.

Accordingly, referring to FIG. 5A again, in another aspect of thesetting monitoring processing, when the threshold V_(th) has not beenset, the processing may be similar to the above, and when the thresholdV_(th) has already been set, as indicated by dotted lines in the figure,the process is standing by until the detected number of times of blinksC_(t) exceeds a predetermined number of times C_(ts) (step 17), andduring this, the eyelid state index value V_(i) is recorded in the blinkdetection processing of FIG. 4. Notably, when the eyelid state indexvalue is the ocular potential as above, the predetermined number oftimes C_(ts) may be, for example, 500 or the like. Then, whenC_(t)>C_(ts) is completed, performance of the setting processing of thethreshold V_(th) as above is instructed (step 15) and the threshold isset through the processing in FIG. 5B using the chronological data ofthe eyelid state index value having been accumulated until C_(t)>C_(ts)is completed, and after the threshold V_(th) is set, the number of timesof blinks C_(t) is reset to be zero (step 16). Afterward, performance ofthe setting processing of the threshold V_(th) may be instructed everytime when C_(t)>C_(ts) is completed, thereby, a new threshold V_(th) maybe determined, and the threshold used in the blink detection processingof FIG. 4 may be updated.

As above, in the present embodiment, briefly stated, in theconfiguration of detecting, as blink occurrence, an occasion when achange width of the eyelid state index value from the baseline exceedsthe magnitude of the threshold from the baseline as described above, thenumber of times of tentative blink occurrence is counted while atentative threshold being changed in the chronological data of theeyelid state index value, a histogram of the number of times oftentative blink occurrence with respect to the tentative threshold isgenerated, and in the histogram, a tentative threshold at which changein the number of times of tentative blink occurrence is minimal is setto be the threshold for detecting blinks. The reason for availability ofthis setting technique of the threshold is that the waveform of theeyelid state index value has the feature that amplitudes of waveformparts in which blinks occur are substantially uniform even when thebaseline varies, and are larger than variation amplitudes of the ocularpotential other than blinks which variation amplitudes overlap with thebaseline by a certain width or more. Further, it is specially noted thatin the technique of threshold setting in the present embodiment, athreshold is not needed to be preset before the start of blink detectionprocessing, and the threshold can be set and updated in proper timingafter the start of blink detection processing. According to such aconfiguration, since when detection of blinks of a driver who is drivinga vehicle, detection of blinks of a subject who is working at a desk, orthe like is performed, the threshold can be updated to be a proper valuein proper timing even when a baseline varies or a change width of anindex value in blink occurrence varies after the start of blinkdetection processing, more highly accurate detection of blinks can beperformed consecutively for a long time without the blink detectionprocessing suspended due to the update of the threshold.

While the description above has been made regarding an embodiment of thepresent disclosure, many modifications and alterations thereof can beeasily achieved by the skilled in the art, to whom it will be apparentthat the present disclosure is not limited to the embodiment exemplarilyshown above but can be applied to various apparatuses without departingfrom the concept of the present disclosure.

What is claimed is:
 1. A blink detection apparatus comprising: an ocularpotential measurement unit having a pair of electrodes, the ocularpotential measurement unit configured to sequentially measure eyelidstate index values indicating a state between an opened state and aclosed state of eyelids in an eye of a subject; and a signal processingdevice, wherein the signal processing device is configured to determine,for each measured eyelid state index value, that a blink occurs when achange of the measured eyelid state index value from a baseline that isa reference value of the eyelid state index value in a case where theeyelids of the subject are in the opened state exceeds a magnitude of athreshold, to store the sequentially measured eyelid state index values,to count a number of times of tentative blink occurrence with respect toeach of a plurality of tentative thresholds, the number of times oftentative blink occurrence being the number of times when the change ofthe eyelid state index value from the baseline exceeds a magnitude of arespective tentative threshold, for each of the eyelid state indexvalues that are sequentially measured and stored over a predeterminedperiod, the magnitudes of the tentative thresholds being different fromone another, to generate a histogram of the numbers of times oftentative blink occurrence with respect to the tentative thresholds, andto set the threshold to a tentative threshold among the tentativethresholds at which a change curve of the histogram is minimal, thechange curve representing change in the number of times of tentativeblink occurrence measured between consecutively applied tentativethresholds in the histogram.
 2. The blink detection apparatus accordingto claim 1, wherein: the predetermined period is a length of apredetermined time; and the signal processing device is configured toreset the threshold for each lapse of the predetermined period.
 3. Theblink detection apparatus according to claim 1, wherein: the signalprocessing device is configured to reset the threshold for each lapse ofthe predetermined period.
 4. The blink detection apparatus according toclaim 1, wherein: the tentative threshold at which the change curve isminimal is a tentative threshold at which a standard deviation in thenumber of times of tentative blink occurrence with respect to apredetermined number of adjacent tentative thresholds in the histogramis minimal, among the tentative thresholds.
 5. The blink detectionapparatus according to claim 1, wherein: the ocular potentialmeasurement unit is configured to measure an ocular potential of thesubject; and the eyelid state index value is the ocular potential. 6.The blink detection apparatus according to claim 1, wherein: the ocularpotential measurement unit is configured to capture an image of the eyeof the subject and to detect a degree of opening of the eyelids in theimage of the eye; and the eyelid state index value is an index valueindicating the degree of opening of the eyelids in the image of the eye.7. A blink detection apparatus comprising: an ocular potentialmeasurement unit having a pair of electrodes, the ocular potentialmeasurement unit configured to sequentially measure eyelid state indexvalues indicating a state between an opened state and a closed state ofeyelids in an eye of a subject; and a signal processing device, whereinthe signal processing device is configured to determine, for eachmeasured eyelid state index value, that a blink occurs when a change ofthe measured eyelid state index value from a baseline that is areference value of the eyelid state index value in a case where theeyelids of the subject are in the opened state exceeds a magnitude of athreshold, to store the sequentially measured eyelid state index values,to count a number of times of tentative blink occurrence with respect toeach of a plurality of tentative thresholds, the number of times oftentative blink occurrence being the number of times when the change ofthe eyelid state index value from the baseline exceeds a magnitude of arespective tentative threshold, for each of the eyelid state indexvalues that are sequentially measured and stored over a determinedperiod, the magnitudes of the tentative thresholds being different fromone another, to generate a histogram of the numbers of times oftentative blink occurrence with respect to the tentative thresholds, andto set the threshold to a tentative threshold among the tentativethresholds at which a change curve of the histogram is minimal, thechange curve representing change in the number of times of tentativeblink occurrence measured between consecutively applied tentativethresholds in the histogram, wherein the determined period is a perioduntil the number of times of tentative blink occurrence that isdetermined by the signal processing device reaches a determined numberof times; and wherein the signal processing device is configured toreset the threshold for each lapse of the determined period.